There are mechanisms in CDMA mobile devices to save battery power while operating within areas having good coverage and areas in which there is no coverage. In areas having good coverage, or areas where relatively strong RF signals are present, mobile device battery power is conserved by entering a sleep mode using the slot cycle index, as described in the CDMA standard, while the mobile device is in an idle state. The slot cycle index is well known to those of skill in the art, and is briefly discussed later. In areas where there is no coverage, the mobile device can enter a deep sleep mode during which it can occasionally ‘wake up’ to check for a presence of RF signals.
Prior to the discussion of the slot cycle index, a brief description of the acquisition sequence of mobile devices follows. When the mobile device is powered up, it enters a search mode to find a pilot channel. The pilot channel is used to establish an initial communications link with a base station. Then the device switches to a synchronisation channel to obtain setup data such as system and network identification information, timing information and information to find a paging channel, for example. Once the paging channel is acquired, the mobile device can remain in the idle state and subsequently enter an access state for registration with the network, for receiving incoming calls, transmitting outgoing calls, or for sending short message service (SMS) data burst messages. The mobile device can then enter a traffic state for receiving incoming or transmitting outgoing calls, or for sending SMS data burst messages.
The slot cycle index operates in the paging channel of the mobile device, and is shown graphically in FIG. 1. In the slotted mode of operation, the mobile device is set to wake up from a sleep mode at predetermined intervals 20, such as every five seconds for example. The device wakes up for a short window of time 22 to receive any message from a base station, which would only send messages during these short windows of time 22 since it is synchronized with the mobile device. While the mobile device is technically ‘on’ all the time unless turned off by the user, the mobile device consumes much less battery power during intervals 20 than during wake up periods 22.
In addition to situations where the mobile device is in a good RF coverage area or no RF coverage area, there are situations in which RF conditions are less than ideal and can cause the mobile device to repeatedly lose the paging channel. Geographical location and network/system coverage are examples of situations in which RF conditions can deteriorate. When the paging channel is, lost, the mobile device enters a search mode to re-acquire the pilot channel, the synchronisation channel and the paging channel. However, because the newly re-acquired signal can be lost again due to the same conditions under which the original signal was lost, the mobile device continues to repeat this re-acquisition process until either RF conditions improve such that the paging channel is not lost, or the mobile device becomes unusable due to excessive drain of the battery. Thus the periodic nature of the slot cycle index and power saving it provides, cannot be maintained. Therefore the mobile device spends most of its time in an active mode instead of a sleep mode, where it expends valuable battery life as the paging channel is frequently gained and lost. While in such RF conditions where the radio signal is not completely lost for a longer period of time, the mobile device is unable to enter any type of sleep mode to save battery consumption.
It is, therefore, desirable to provide a method for conserving mobile device battery power in situations where RF conditions are poor.